This invention relates to the reclamation of waste ink by removal of entrained water, solvents and solid impurities so as to render the ink reusable.
While there are a number of different printing processes that use different types of ink, the ink used in offset printing is especially difficult to reclaim due to the inherent nature of its composition and the nature of its typical contamination. Offset printing is a process in which an inked impression from a plate is first made on a rubber-blanketed cylinder and then transferred to the paper being printed. More specifically, the plate clamped onto a rotating plate cylinder comes in contact first with a dampening roller, then an inking roller and finally, the blanket cylinder. The dampening roller wets the plate with water or fountain solution so that the non-printing areas will repel ink. Ink picked up from the inking roller is transferred to the rubber blanket on the blanket cylinder. Paper is printed as it passes between the blanket and impression cylinders. Excess ink not transferred in the printing process is collected as waste, is substantially contaminated and, hence, is not reusable as is. The waste ink typically contains a very large amount of water, some solvents and a significant amount of paper dust and lint.
The motivation for reclaiming this waste ink is two-fold. Virgin ink is expensive and the amount of waste ink produced in a typical offset printing operation is significant. Furthermore, as waste ink is toxic and considered hazardous waste, disposal thereof is becoming an increasingly difficult and expensive undertaking. Hence, a cost-effective reclamation of waste ink is both economically advantageous and environmentally desirable.
Numerous reclamation systems have been conceived. SSome have been implemented, but none have proven very successful. The nature of the ink, in combination with its typical contamination, makes for a very difficult material to purify. Ink used in the offset printing process has a very high pigment content which necessarily implies a correspondingly low oil content and therefore high viscosity. Paper dust and lint picked up during the printing process absorbs some of the oil and thereby renders the ink even more viscous. Entrainment of water has the effect of further thickening the ink. What results is a very stiff and viscous intimate admixture of mineral oil, pigment, water, paper dust and lint, and solvents. Ideally, the ink should be separated out with no significant reduction in its pigment content and with the rheological properties of virgin ink.
A much tried method of separation is one that employs the use of a series of strainers and filters. The high viscosity of waste ink hampers such a process and therefore requires dilution with virgin ink, an expensive and somewhat counterproductive step. The high content of very fine paper particulate rapidly clogs any fine mesh strainers or depth cartridges and therefore requires frequent cleaning and/or replacement cycles. Furthermore, a layer of paper particulate on a strainer or filter cartridge not only substantially increases back pressure, but in effect serves as an even finer mesh filter that is capable of actually restraining the ink pigment. Such an effect is herein referred to as "pre-filtration." Increasing the system's pressure will only aggravate this problem. Neither strainers nor filters can remove any entrained water from the waste ink and therefore, the end result of such a method is an inferior ink with a high water content that must be blended with a considerable amount of virgin ink to make reuse thereof feasible. Any replaceable filter elements must, of course, be properly disposed of.
Another approach entails the use of a centrifugal method. Once again, the nature of this process requires that dilution or blending with virgin ink be undertaken, both before and after the centrifugal separation. Further disadvantages of this process focus on the centrifuge itself, i.e., the extremely high cost of the equipment, the degree of skill required to operate it properly and the labor intensive manipulation thereof. In order to effectively take advantage of the specific gravity difference between the waste ink components and contaminants, an acceleration of many thousands of times the force of gravity is required. This calls for rather sophisticated high-speed equipment and close attention to its operation. At such high g-loads, even slight imbalances must be avoided, the viscosity of the waste ink must be constantly monitored and adjusted to ensure a smooth introduction into the device, and ink contamination levels must continually be measured so that proper throughput rates can be estimated and the equipment fine-tuned accordingly. Contamination levels of the reclaimed product must again be measured after centrifuging to determine whether a proper separation was accomplished. At typical particulate levels, frequent shut-down is required to disassemble the centrifuge to facilitate clean-out of the centrifuge bowl which entails a labor-intensive and messy procedure. An intrinsic disadvantage of the centrifugal method is that a portion of the ink pigment is separated out and, hence, lost. This is due to the fact that specific gravity of the pigment is higher than that of the oil base. The end product is an ink with a significantly reduced pigment content that must be blended with substantial amounts of virgin ink to be usable in the offset process.
The described disadvantages of the prior art are overcome by the present invention.